1. Field of the Invention
The present invention relates to a non-aqueous cell. In particular, the present invention relates to a non-aqueous cell having a specific structure which can maintain safety while increasing capacity.
2. Prior Art
Non-aqueous secondary cells, typical examples of which are lithium ion secondary cells, comprise organic solvents as major solvents of electrolytes, and they have a large capacity, generate a high voltage and a high output, and achieve a high energy density. Thus, demands for non-aqueous secondary cells have increased.
Nowadays, lithium ion secondary cells, which comprise LiCoO.sub.2 as a positive electrode active material and a carbonaceous material as a negative electrode active material, are commercialized. Different from conventional non-aqueous secondary cells which use a lithium metal as a negative electrode, to increase a capacity, such lithium ion secondary cells comprise positive and negative electrodes, which are formed by applying the pastes of the above active materials dispersed in organic solvents together with binders onto the both surfaces of positive and negative electrode collectors, respectively, to form coating films containing the respective active materials. The band-form electrodes are spirally wound with insertion of a separator between them, and the wound electrode member is installed in a cell can to assemble a cell.
Non-aqueous secondary cells use, as a major solvent of electrolytes, mixtures of cyclic esters (e.g. ethylene carbonate, etc.) and esters (e.g. dimethyl carbonate, methyl propionate, etc.), all of which are flammable solvents. Thus, cells are very carefully designed to provide good safety of the cells. Thus, conventional non-aqueous cells have an interruption vent in a sealing plate to prevent the explosion of cell cans caused by the generation of gasses, a PTC device in a cell can to prevent heat generation due to the flow of an over-current, or a shutdown mechanism which prevents the migration of lithium ions by clogging micropores of separators by the fusion of the micropores at high temperatures.
However, it has been found by the study of the present inventors that non-aqueous cells may not have sufficient safety when a cell capacity should be further increased or when the cells should follow various specifications presented by users, unless the structure of a power-generating element is improved. That is, it has been found that conventional non-aqueous cells tend to exhibit insufficient safety in safety tests under severe conditions such as a crush test, a nail penetration test, or an external short-circuiting test, all of which intentionally simulate the abnormal use of cells.
For example, a crush test simulates a situation such that a cell is accidentally crushed. It has been revealed in the crush test that a cell is easily short-circuited when a lead member, which is welded to a negative electrode collector, faces a positive electrode through a separator. A reason for such a result may be that the lead member for a negative electrode breaks a separator when a cell is broken by pressing. When one electrode is in contact with a cell can which functions as a terminal for the other electrode in the course of a crush test, a short circuit current flows, and thus the amount of generated heat increases, if a coating film containing an active material with a high resistance is present. When a separator surrounding an electrode member melts by such heat, other parts of a positive electrode maybe in contact with the cell can, and thus a secondary internal short-circuiting may form. Furthermore, in the case of a non-aqueous secondary cell having an electrode which comprises a coating film containing an active material on a collector, foreign materials such as metal pieces from the production processes are often present in cells, or the active material often drops when an electrode member is installed in a cell can. In general, a positive electrode and a negative electrode are separated with a separator. Thus, such foreign materials rarely form short-circuits. However, when the size of foreign materials is large, so-called minute short circuits (soft short) form in a crush test, if such foreign materials are present in a cell can, and may finally trigger the formation of internal short circuits.
A nail penetration test can surely form a short circuit in a smaller region than the crushing of a cell or an external short-circuiting. Thus, a current concentrates at the short-circuited region, and thus such a region is more easily heated so that the cell is partly heated to a high temperature quickly. Therefore, a separator tends to be unevenly fused (clogging by fusing). In addition, an amount of heat, which is generated by the reaction of an electrolyte and a negative electrode in the short-circuited region, increases. Accordingly, the nail penetration test is a very severe safety test which can find the lack of safety that would not happen under normal service conditions. Thus, it is supposed that cells can maintain safety in the case of abnormal use, when the safety of cells is confirmed by a nail penetration test.
A nail penetration test at a high temperature of 45.degree. C. can heat a cell to a higher temperature and more easily causes a thermal runaway reaction in a cell than a nail penetration test at a room temperature. Furthermore, when a nail is halfway penetrated, for example, a half of a nail is penetrated, a short-circuited region is small and thus a current is further concentrated to easily generate heat. Consequently, a nail penetration test at 45.degree. C. with piercing a half of a nail into a cell is a very severe test to check the safety of a cell. It is believed that cells have sufficient safety in practical use, once the safety of cells is approved by a safety test under such severe conditions.
In addition, non-aqueous cells are subjected to a comprehensive external short-circuiting test, in which a cell is charged to 4.2 V or larger, and then positive and negative electrodes are connected, to confirm the high safety of the cells. With the increase of the energy level of cells, a larger current flows through a cell on external short-circuiting. Thus, a separator, which is in contact with a part having a relatively high resistance in a cell, is melted, so that secondary internal short circuits are induced, and the cell tends to be partly heated to a high temperature quickly. Like in the above crush test, when a separator between a wound electrode member and a cell can is melted, one electrode and the inner wall of a cell can which functions as the other electrode are brought into contact with each other to form a short circuit. Furthermore, when foreign materials, which have a high resistance, are present in a cell can, minute short circuits grow to short circuits. As a result, heat may be locally generated in regions in which short circuits form. Thus, it is believed that cells can maintain safety when they meet with abnormal use, once the safety of cells is approved by an external short-circuiting test.
An external short-circuiting test at a high temperature of 45.degree. C. can heat a cell to a higher temperature and more easily causes a thermal runaway reaction in a cell than an external short-circuiting test at a room temperature. Consequently, an external short-circuiting test at 45.degree. C. is a very severe test to check the safety of a cell, and it is believed that cells have sufficient safety in practical use, once the safety of cells is approved by a safety test under such severe conditions.
The energy density of a cell is further increased with the recent trend of the increase of a capacity. Thus, it is necessary for a cell to have excellent safety in a crush test, a nail penetration test and also an external short-circuiting test, all of which are safety tests under severe conditions. To this end, the internal structure of a cell should be modified to a structure which can hardly ignite.